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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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458                               ALASKAN GLACIER STUDIES
the current of the Copper River, are responsible for this great accumulation of coarse, outwash gravels. The river is divided into innumerable, branching, shifting channels of the type which develop in heavily-loaded glacial streams whose current is checked (PI. CLXXIX).
The Former Copper River Fiord. The third section of the course of the Copper River across the Chugach Mountains is between Childs Glacier and the railway bridge at Flag Point. This is the mouth of the Copper River canyon, which flares so much in a distance of twelve miles that the width of four miles at Childs Glacier increases to thirteen miles at Flag Point. The mountain walls attain heights of 4000 to 5000 feet within 1 to 3 miles of the valley bottom.
The lower portions of the valley wall slope very steeply as a result of glacial erosion. In a cliff just south of Miles Glacier on the eastern valley wall there is an ascent of 1700 feet in a distance of 2000 feet, and just north of Flag Point on the western valley wall there is an ascent of 4000 feet in less than a mile.
The tributary valleys enter at discordant grades, as near McPherson Glacier where extensive glacial erosion is shown by the hanging valley over the end of which this ice tongue cascades. The hanging cirque between McPherson and Miles Glaciers has been eroded by a local glacier now melted away. The valley in which Fickett Glacier lies seems to have been much deepened and steepened in the lower slopes by ice erosion. The Childs, Goodwin, and Johnson Glaciers probably occupy hanging valleys, but is not certain whether the valley occupied by Martin River Glacier is discordant or not.
It is thought that this lower, flaring portion of the Copper River canyon is a former fiord and that the waters of the Pacific Ocean formerly extended at least as far north as the Childs and Miles Glaciers.
This conclusion is based upon the fact that borings at the mouth of the canyon near Flag Point, close to the western valley wall, and at Hotcake Channel, about in the middle of the valley, show that there is no rock for a distance of at least 25 to 35 feet below sea level. At the head of the flaring section of the valley the soundings in the Miles Glacier lake show that the rock bottom is over 184 feet below sea level.
From these facts it may be concluded that when the trunk glacier of the Copper River canyon retreated it had deepened this flaring embayment so that the lower fifteen miles or so were below sea level. That the ice did not halt long enough to fill this embayment at once is suggested by the absence of recessional moraines. That the ocean then extended up to or above present sea level is shown by the presence of wave-cut forms, like the stacks at and west of Alaganik. If the water extended inland to the stacks at Alaganik it must have also filled the Copper River fiord, at least as far north as Miles and Childs Glaciers.
The flaring form of the valley walls indicate that this former fiord had outlines similar to Orca Inlet, Sheep Bay, Port Gravina and several other fiords to the west, which would be readily transformed to filled fiords exactly like the mouth of Copper River canyon if great glacial streams flowed into their upper portions.
The Copper River Delta. The Copper River has not only completely filled this former fiord, but it has built a delta (PL XC1H) which extends about 12 miles out into the sea south of Flag Point and shallows the Pacific Ocean for twenty miles more. This delta coalesces with the alluvial fans of Sheridan, Scott, and adjacent glaciers, on the